I used to use a "masons" chisel with a 5lb hammer to chop rivets off frame brackets...until I saw a guy at the junkyard use a all steel camping hatchet and a hammer!--he put the hatchet between the bracket and the frame,and gave it 3 or 4 good whacks,and the rivets sheared right off,the brackets were not damaged hardly at all(nothing bolting them to another frame would not fix anyway)..
I was amazed how good it worked!--I'd wear a face sheild if you try this at home!..one reason it works well is because the handle of the hatchet keeps your hand out of the line of fire,and your not scared to give a good blow with the 5lb hammer!..
I took this idea one step higher!--I had an old logspilitting wedge that had an "eye" in the center for a handle,like a hammer head does--I welded a peice of pipe to it 18" long for a handle,and I hit it with a 5lb ball pein or sledge hammer HARD and it only takes one or two whacks to shear the rivets!--If you use it like an AX its pretty effective too--I find it invaluable for "demolition" of vehicles your cutting up for scrap!--sheet metal is quicky cut away from body bolts,and sheet metal screws with boogered heads are no problem..just use "Thor's Hammer" and they go away FAST!..
Of course,a torch works nicely too,but oxogen is expensive,and cant be bought here on weekends(place is closed sat and sun!)

..so brute force prevails where torches have failed!..